Dave,
You cannot "drop the certification" as you suggest and then use the R22 in a commercial training operation.
The R22 could be modified legally by the suplemental type certificate (STC) process to make a better trainer or any purpose. Maybe someone should consider that.
I just read about a new company that is building Bell 206 blades with a STC (Bell is not happy of course and will try to stop the STC holder).
slowrotor